This one is surprisingly easy. The first thing you have to do is get an eight year old friend. We'll call him, oh, Kurt. It helps if you're also eight years old but that's not necessary. Then find a barn, preferably an old barn with attached side buildings used to store tack and channel calves through to get them into a trailer for auction. Make sure you can climb up a rickety wooden ladder and get up to the roof of one of the side buildings. The roof should be about twelve feet high.
Once you've secured all this preliminary equipment, get a small tree frog. The tree frog is really the critical part of this sequence of events. Don't skimp on the frog.
Next, take the frog and your friend Kurt up the ladder, walk out onto the roof and set the frog at the edge. The frog will likely do what it is that frogs do. If it doesn't, poke it in the rump with your finger. Lean forward to watch the frog fall.
(I know what you're thinking. But this is not quite the part about falling off the roof yet. Wait for it.)
Squeal with maniacal eight year old glee and swing down to the barnyard on a handy Douglas Fir branch that is perfectly positioned as a method of reaching the ground quickly and safely. Retrieve the frog.
Climb back up on the roof and repeat the process, only this time letting your friend Kurt push the frog. Squeal with glee. Retrieve the frog.
At this point, since it is clearly your turn to make the frog jump off the roof, you should ensure that you are the one to retrieve the frog. Failure to accomplish this simple task will inevitably lead to the following:
Kurt gets the frog and claims that he's going to take your turn. You don't want to believe him but since possession is nine tenths of the law you realize that you could very soon get screwed out of what is rightfully yours. You follow Kurt up onto the roof and see his happy face turn back to you and repeat that he's going to be the one to push the froggie off the roof and that he's apparently going to delight in intentionally stealing your turn. You explain in brief, eight year old language that you disagree with this course of action, but are powerless to do anything about it because you're watching Kurt run away from you across the roof. You chase him.
Here's where things get interesting. You watch Kurt turn his head and check where the edge of the roof is. He then turns back to resume taunting you while still running at a pretty good clip. You follow, still pleading your case but beginning to accept that as he gets closer to the edge it is more likely that he will, in fact, take your turn. At this point your heart begins breaking because you really, really wanted to push the froggie off the roof and Kurt has done stuff like this before so you're really at a loss for what to do and you begin to reevaluate your position in the universe and wonder how God could be so cruel.
Watch, dumbstruck, as Kurt runs right off the roof at full speed. Stop running and go to the edge to see Kurt sprawled out on the dirt, sucking in air for a pretty good cry if he could ever get it out. Swing down, using the quick and safe method described above, and stand helplessly as Kurt flops about like a dying fish. Run to go get the babysitter. Spend the next couple of hours in the Emergency Room, telling this same story to several different people.
And that's how to fall off a barn roof.